1. Field of the Invention
The present invention relates to an apparatus for automatically adjusting channel box fitting position for use in lowering and fitting a channel box in form of a square pipe onto a nuclear fuel assembly under water in a fuel reservoir. The nuclear fuel assembly is to be placed as heat source in a reactor vessel of a boiling water reactor (BWR) plant, and the channel box is fitted on the fuel assembly for promoting contact between the fuel assembly and precooling water.
2. Description of the Prior Art
In the BWR plant as shown in FIG. 7, the fuel reservoir G is in continuation with the reactor vessel E through a communicating passage F. Referring also to FIG. 1, the function of the reservoir G is to provide temporary storage for consumed or half consumed nuclear fuel assemblies A and their channel boxes, and the distance between a floor surrounding the reservoir and top surfaces of the fuel assemblies A deposited in the bottom of the reservoir is about eleven meters, the reservoir being filled with water up to 50 cm to one meter below the floor. Transfer of the fuel assemblies A (about 4 meters long) from the reactor vessel E to the reservoir G and mounting and dismounting of the channel boxes B (about 4 meters long) are all carried out under water, in depths of at least 1.5 meters. The underwater mounting and dismounting of the channel boxes B are conducted for the purpose of inspection of the fuel assemblies A taken out of the reactor vessel E to check on spacing among a plurality of pipy fuel rods contained in each fuel assembly, their deformation and so forth by using underwater television cameras. The fuel assembly A taken out of the reactor vessel E by a conveyer J is placed on an immersed elevator K provided on a side of the reservoir G. The elevator K is raised till the top of the fuel assembly A is about 1.5 meters below the water surface. Then a poolside operation is carried out to loosen a screw holding the channel box B to the fuel assembly A through one of gusset plates b at two top corners of the box B and to allow a hook shaped rod to catch the gusset plates b of the channel box B. The fuel assembly A comes off the channel box B downwardly as the elevator K is lowered. The channel box B is shelved onto an underwater rack provided beside the elevator, and the elevator is raised again to bring up the fuel assembly A to an appropriate height for inspection by the underwater television cameras.
After the inspection the channel box B is fitted back onto the fuel assembly A. This operation involves fitting of the channel box B about four meters long to the fuel assembly about four meters long in a depth of about ten meters below the water surface. In other words, the channel box B is fitted to the fuel assembly A lowered where its top is at least six meters deep. The channel box has cross sectional dimensions substantially corresponding to the 13.times.13 cm top surface of the fuel assembly A, and it is a very difficult operation to fit the channel box from above to the top of the fuel assembly at six meters below the water surface smoothly and with the respective four corners in concurrence.
In order to overcome the above noted difficulty, the present inventor developed his basic invention, namely an apparatus for automatically adjusting channel box fitting position as disclosed in a Japanese patent application laid open under No. 55-80094, whose principal part is shown in FIG. 8. This apparatus is capable of automatically adjusting the position of the channel box B' for fitting onto the top of the nuclear fuel assembly A' disposed at a depth of six meters below the water surface. The apparatus comprises a hanger cap 1' removably attachable to a hanger H' provided on an upper tie plate T1' of the nuclear fuel assembly A' and a pair of guide rollers 2' mounted on the hanger cap 1' rotatable on axes extending diagonally of the tie plate T1' when the hanger cap 1' is attached to the hanger H' in plan view and inclined downwardly toward outer ends thereof in side view. The guide rollers 2' are adapted to rotate by contact with bottom inner walls of the channel box B' to a correct position for fitting onto the fuel assembly A'.
In this apparatus the guide rollers 2' are shaped to flare along the axes of rotation toward the outer ends thereof and each of the guide rollers 2' is supported at two points by frames 4' extending from the hanger cap 1'. On the other hand, the channel box B' carries space regulator members or spacers 20' about 3.5.times.5.times.0.6 cm caulked by rivets 20 to two top outer corners thereof (see FIG. 1). When setting in the reactor vessel, the spacers 20' contact a position and space retainer grid (not shown: each grid accommodates four fuel assemblies) disposed in the reactor vessel, to avoid direct contact of the channel box B' with the grid. In the case of a fuel assembly giving big power output or a different type of fuel assembly, its channel box may have heads of the caulking rivets 20 projecting inside the corners of the channel box. Therefore, the prior art apparatus, which requires large occupying spaces in corners of the channel box B' because of the large outer ends of the guide rollers 2' and the presence of the frames for supporting the outer ends of the guide rollers, has a disadvantage of being applicable only to channel boxes having the space regulator members or spacers and caulking bolts away from the corners.
While the guide rollers 2' are inclined downwardly along the axes of rotation toward the outer ends thereof to provide desired downward guidance for the channel box B', the shape of the guide rollers 2' flaring toward the outer ends offers considerable resistance to the downward movement of the channel box B'. Therefore the prior art construction as described is unsatisfactory with regard to smooth guidance.